## Core Testing Philosophy

### Test-Driven Development (TDD)

Always follow the TDD cycle:

1. **RED**: Write a failing test for the desired behavior
2. **GREEN**: Write minimal code to make the test pass
3. **REFACTOR**: Improve code while keeping tests green

```python
# Step 1: Write failing test (RED)
def test_add_numbers():
    result = add(2, 3)
    assert result == 5

# Step 2: Write minimal implementation (GREEN)
def add(a, b):
    return a + b

# Step 3: Refactor if needed (REFACTOR)
```

### Coverage Requirements

- **Target**: 80%+ code coverage
- **Critical paths**: 100% coverage required
- Use `pytest --cov` to measure coverage

```bash
pytest --cov=mypackage --cov-report=term-missing --cov-report=html
```